Researchers at NASA Goddard Space Flight Center have embarked on a program to study the formation and growth of cosmic grains. This includes experiments on the homogeneous nucleation of refractory vapors of materials such as magnesium, lead, tin, and silicon oxides. As part of this program, the Chemical Engineering Department of the University of Virginia has undertaken to develop a math model for these experiments, to assist in the design and construction of the apparatus, and to analyze the data once the experiments have begun. Status Reports 1 and 2 addressed the design of the apparatus and the development of math models for temperature and concentration fields. The bulk of this report discusses the continued refinement of these models, and the assembly and testing of the nucleation chamber along with its ancillary equipment, which began in the spring of 1988

Ferguson, F. T.

Lilleleht, L. U.

Stephens, J. R.

English

NASA Technical Reports Server

' I
Status Report III /_ _'....._-__'-
GrantNo.NAG-5-865 /_p
December 1, 1987 - August 31, 1988
MICROGRAVITY NUCLEATION AND PARTICLE
COAGULATION EXPERIMENTS SUPPORT
Submitted to:
_¢- National Aeronautics and Space Administration
Goddard Space Flight Center
Greenbelt, MD 20771
Attention: Dr. Joseph A. Nuth
Code 690
Laboratory for Extraterrestrial Physics
Space and Earth Sciences Directorate
Submitted by:
L. U. Lilleleht
Associate Professor _ I
F. T. Ferguson
Graduate Assistant
1
J. R. Stephens
Matrix, Inc. }
DEPARTMENT OF CHEMICAL ENGINEERING
UNIVERSITY OF VIRGINIA
CHARLOTTESVILLE, VIRGINIA 22901
1989001685
https://ntrs.nasa.gov/search.jsp?R=19890001685 2020-03-20T05:48:51+00:00Z
Status Report III
Grant No. NAG-5-865
December I, 1987 - August 31, 1988
; MICROGRAVITY NUCLEATION AND PARTICLE
COAGULATION EXPERIMENTS SUPPORT
!
Submitted to:
l
| National Aeronautics and Space Administration
Goddard Space Flight Center
Greenbelt, ND 20771
Attention: Dr. Joseph A. Nuth
Code 690
Laboratory for Extraterrestrial Physics
Space and Earth Sciences Directorate
Submitted by:
! L.U. Lilleleht
_" Associate Professor
i F. _. Ferguson
: Graduate Assistant
i J.R. Stephens
Matrix, Inc.
Department of Chemica] Engineering
SCHOOL OF ENGINEERING AND APPLIED SCIENCE
UNIVERSITY OF VIRGINIA
CHARLOTTESVILLE, VIRGINIA
Report No. UVA/528260/CHE89/I03 Copy No.
!_ October 1988
- WI
_ TABLE OF CONTENTS
i
I. Summary 1
II. Introduction 2 i
III. Research Accomplishments 3 |i
A. Model Development 3
B. Chamber Assembly 4
C. Data Acquisition System 6
IV. Future Work 8
....' ...... "_ 1989001685-003
1I- SUMMARY
P A number of refinements have been incorporated in our finite difference model for
W the developing temperature and supersaturation profiles in the nucleation chamber.
These include: conduction through the chamber walls, the effect of various thermal
_ breaks such as joints and seals, and improved method of accounting for thermal
radiation. In addition, the programs have been converted to using an alternating-
_, direction implicit (ADI) method to make them more efficient to run (University of
i Virginia).y-
Major effort during the summer was devoted assembling the nucleation chamber
and its accessories. Preliminary tests have shown that the heaters are capable of
producing temperatures well above the necessary minimum. "Smoke" has been pro-
duced with zinc; however, attempts with magnesium vapors have not been successful
in these initial trials, most probably due to oxide formation on the molten metal
surface (Goddard Space Flight Center and the University of Virginia)•
Work has continued on defining the experimenta] configuration and development
of the software for the data acquisition system. Recommendations for additional
hardware and software have been submitted to allow the development of a more
robust and capable overall system (Matrix, Inc.).
1989001685-00a
2 [:1
II- INTRODUCTION
f-
Researchers at NASA Goddard Space Flight Center have embarked on a program
to study the formation and growth of _:,osmicgrains. This includes experiments on the
homogeneous nucleation of refractory vapors of materials such as magnesium, lead, tin
and silicon oxides. As part of this program, the Chemical Engineering Department of
the University of Virginia has undertaken to develop a mathematical model for these _
experiments, to assist in the design and construction of the apparatus, and to analyze
t.
the data once the experiments have begun.
(. Status Reports I and II addressed the design of the apparatus and the development "
of mathematical models for temperature and concentration fields. The bulk of this
,!
report discusses the continued refinement of these models, and the assembly and
testing of the nucleation chamber along with its ancillary equipment, which began in
the spring of 1988.
Ji
1989001685-005
III- RESEARCH ACCOMPLISHMENTS
A. Model Development
Most of the early spring was spent on improving the programs for the temperature
and supersaturation profiles. In Status Report II, plots were given for the tempera-
ture distribution and supersaturation ratios for several different operating conditions.
The program which modeled the temperature distribution ir.:luded the effects of con-
duction through the containing aluminum wall, conduction through the ambient gas,
and radiation heat, transfer. This early version of the program used an explicit fi-
nite difference method to solve the equation of energy for the temperature at various
nodes. It took a significant amount of time to run, though, because it used an explicit
method to solve the finite difference equations. The time step in this method had tob
be limited to rather small values because of problems with stability. The program
for the supersaturation ratio used the same method, but since the transient data are
only needed for a 20-second real time period, the small time step problem was not as
critical in that program.
i. During the spring, these temperature distribution and supersaturation programs
were converted to using an implicit finite difference method, namely the alternating-
direction implicit or ADI method. Although the ADI method is computationally more
¢.
difficult, and takes more time to sweep through a single iteration, it has the distinct
advantage that it is stable for any size time step. Of course, the choice of the time
I,. step will affect the accuracy of tile solution, but the implicit time step can usually
be much larger than that required by the explicit method. In our case, we were able
Q
1989001685-006
4to use a time step on the order of 125 times larger, thus making the program much
more efficient to run.
Status Report II also mentioned the possibility of modeling convection within the
nucleation apparatus. This would involve the simultaneous solution of the equations
governing the velocity and temperature fields within the chamber. After completing
the work on the temperature and supersaturation programs, we began to tackle these
.:
equations. We started with a relatively simple geometry so that our method of so-
lution could be verified with published work. The results have been very close, but
there are still some slight discrepancies that need to be worked out before we attempt
to model the natural convection process in the nucleation apparatus, thoagh.
B. Chamber Assembly
The nucleation experiments will be conducted in a specifically designed cylindrical
chamber. A set of resistive heaters centered at the top of the chamber will gener-
ate refractory vapors which will then be allowed to diffuse downward through the
temperature gradient imposed by these heaters. The downward diffusion direction
was chosen to reduce convection interference with the nucleation process. When the
evolving vapor concentration profile exceeds the critical supersaturation ratio, nucle-
t
ation and condensation will commence as evidenced by the formation of a particulate ',
"smoke" cloud. A computer- based data acquisition system will monitor the nucle- i
ation conditions while a video camera records the location of the smoke front. After t
initial laboratory tests, the experiment will be prepared for runs aboard NASA's
i
h:C-135 research aircraft capable of flying a pattern with a sequence of 20-30 second
I
1989001685-007
periods of microgravity conditions.
The resistive heaters for the nucleation apparatus arrived from the febricator in
January, 1988. During a one week period in January we were able to check these
heaters at the Goddard Space Flight Center and observe their behavior as a function
ef the applied electrical power in a stainless steel bell jar. Some fabrication flaws
were noted which required fabrication of new heaters. Although both heaters also
had some local "hot spots", the measured temperatures seemed to be sufficiently high
for producing vapors of the refractory metals.
Overall assembly of the apparatus began during summer, 1988 at Goddard. Since 1
the equipment needs to be mobile for flights on the KC-135, we mounted the chamber
and its accessories on a flat cart. These accessories included a vacuum pump, two
transformers and variacs, a gas cylinder, and a portable computer and data acquisition
system. We assembled the mare components of the chamber itself and connected the
heaters to the transformers and variacs. We also designed an attachment to the
chamber for holding two lasers which will be used for experiments on scattering from i
f
the condensed smoke particles. This piece of equipment will probably be completed
in October, 1988. t
t
The second half of the summer was spent testing the heaters and attempting i
some initial nucleation runs. Thermocouples were placed within the chamber and !
l
connected to the data acquisition device, and a simple program was written to display
the temperatures of the set of thermocouples. During the heater testing, we were able
to get the crucible up to a maximum temperature of approximately 1100-1300°C. This
was well above the necessary minimum temperature of approximately 750°C. After
1989001685-008
l:ii¸ "7-"............ ................"
6 "!
mounting a shutter assembly to close and open the port to the crucible, we tried
vaporizing magnesium to see if we could get smoke particles tu form. Magnesium
had been selected as one of the first test materials because it has a relatively low
melting point and high vapor pressure. Attempts to produce magnesium vapors were
unsuccessful, even after several trials and using hydrogen, rather than nitrogen, as
the ambient (carrier) gas. Hydrogen was expected to eliminate the formation of oxide
layer on the molten magnesium surface in the crucible. Finally we tried usiug zinc
instead of magnesium. Zinc has a melting point somewhat lower than magnesium
and a much higher vapor pressure at the temperatures we were using. We were able
to produce smoke, but we believe that these smoke particles were actually from ZnO,
rather than Zn metal. The motion of these smoke particles, whether they were Zn
or ZnO, did confirm our suspicion that natural convection could still occur within
the chamber, at least with the thermal operating and boundary conditions of these
preliminary tests.
C. Data Acquisition System
The data acquisition system software package is being de-'eloped ur,der a subcon-
4
tract to J.R. Stephens of Matrix, Incorporated of Santa Fe, New Mexico.
During the above contract period, work continued on defining the experimeutal
configuration and dew.'loping software to acquire data in the laboratory and in the
flight. A configuration for a second computer system, which is to be used predom-
t inantly in the laboratory but covld also serve as a backup for the tlight has been
recommended, Another Compaq Portable I i was recommended with maximum on-
Ii
If
1989001685-009
._ _ I_,il _
-_llp,-_.__':_, ¸_ ....... :i
't
, t
7 "S
board memory and enhanced graphics adaptor. On a visit to Goddard, we attempted
_-, to load and test the Keithley software, but differences between the I(eithley software
and the Compaq Operating System prevented running the test programs developed
during the last contract period. New Keithley software was ordered and now operates
correctly.
Due to the complexity of new software development, it has been recommended
that we plan to use Quick500, a compiled version of the Keithley SoftS00_which makes
use of the advanced capabilities of Microsoft's QuickBASIC. This product will allow
!
much more sophisticated software to be developed in a shorter time, at a lower cost,
and still provide a more robust and capable data acquisition system.
1IF"
i1_¸ ,¸ _ 1989001685-010
¸. .....
8
"1
IV- FUTURE WORK
The oxidation of the molten metal surface layer is probably due to small air leaks
into the chamber. To fix these leaks, the chamber will have to be taken apart and
rechecked to make sure all the seals are working properly. Once this is done, the la_ "
mount and video camera can be attached to the chamber. These pieces of equipmelt
will then be connected to the computer and the electrical wiring changed to make the
chamber and the cart acceptable for flights on the KC-135.
If the oxide formation persists, we may be forced to use hydrogen in place of argon
,.s the ambient gas. Consequently, the computer programs would need to be modified
to reflect this change. This should not present any major difficulties, however. We also
expect to continue modeling the natural convection within tile nucleation chamber.
Observations of smoke movements have shown that convective currents are likely
to occur. O_,r modeling may be able to determine tile significance of convection
at gravities corresponding to, say, lunar or martian environments. Should colwective
effects at lunar or martian gravity levels be tolerable, then we could extend the length
of the experimental runs aboard KC-135 since longer periods of reduced gravity at
these levels are available on that aircraft.
D
1989001685-011


Microgravity nucleation and particle coagulation experiments support

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